Undoubtedly, location information is a fundamental content to be utilized to extract the geographical relationship between the users and the environments so as to further understand the user behaviors. The importance Is and promise of location-aware applications has led to the design and implementation of systems for providing location information, particularly in indoor and urban environments. Currently, there is an increasing market need for high-accuracy tracking of people and assets in real time in many different application scenarios including office, healthcare, coalmine, subway, smart building, restaurant etc. For instance, in office environment, employees are required to access confidential information database in certain secure zone. Out of the zone, any access will be prohibited. The examples of the secure zone can be a single room, part of a working area, and even a table.
The well-known Global Positioning System (GPS) can provide the object's location information at the accuracy of several ten meters outdoors, however, in indoor environment GPS does not work well since the positioning result of GPS is degraded dynamically by multipath effect and signal obstruction.
Conceptually, indoor location system can be categorized into tracking system and navigating system. In tracking system, position calculation runs at server side so as to locate the object's position for tracking. In this case, a database kept track of the locations of all the entities, thereby the user privacy may not be guaranteed. Further, in a tracking system, all control and management functions are centralized at the server side so that it is not possible to deploy and administer a system in a scalable way.
For example, in U.S. Pat. No. 6,216,087 to R. Want entitled “Infrared Beacon Position System”, a proximity based location system “Active badge” is build over bidirectional infrared link where one infrared beacon is deployed in each room and the mobile unit is a small, lightweight infrared transceiver that broadcast an unique ID every a fixed interval. Since infrared signals can hardly penetrate walls, ID broadcasts are easily contained within an office, providing high-accuracy localization at room granularity.
In “Bat” system of U.S. Pat. No. 6,493,649 to Jones entitled “Detection system for determining positional and other information about objects”, users wear small badges which emit an ultrasonic pulse when radio-triggered by a central system. The system determines pulse TOA (Time of Arrival) from the badges to dense receiver array installed on the ceiling, and calculates the 3D positions of the badges based on a multilateration algorithm.
“Sonitor” system of International Patent Publication No. WO 03/087871 A1 to S. Holm entitled “A system and method for position determination of objects” provides an ultrasonic-only indoor positioning system to achieve room-granularity location accuracy. Sonitor's tags transmit 20 kHz to 40 kHz ultrasonic signals to receivers located in the listening area. Through frequency modulation, each tag communicates a unique signal to the receivers, use algorithms to read the signals and then forward their ID to a central server.
On the other hand, navigating system aims to perform position calculation at client side and let the objects know their own physical locations. User applications do not advertise their locations unless they want to be discovered by others. By this means, user privacy concern can be adequately met.
In a non-patent document “RADAR: An In-Building RF-based User Location and Tracking System, P. Bahl etc., Proc. IEEE INFOCOM, 2000”, a location system based on strength of received signals in 802.11 wireless network is presented. The basic RADAR location method is performed in two phases. Firstly, in an off-line phase, the system is calibrated and a model is constructed with strengths of received signals at a finite number of locations distributed about the target area. Secondly, during on-line operation in the target area, mobile units report the signal strengths received from each base station and the system determines the best match between the on-line observations and any point in the on-line model. The location of the best matching point is reported as the location estimate. However, RF (radio-frequency) systems, which use signal strength to estimate location can not yield satisfactory results because RF propagation within buildings deviates heavily from empirical mathematical models.
Additionally, in a non-patent document “The Cricket Location-Support System, B. Nissanka, etc., Proceedings of the Sixth International Conference on Mobile Computing and Networking, Boston, Mass., USA, August 2000”, a “Cricket” system is presented. As shown in FIG. 1, the Cricket system consists of a set of independent, unconnected Ultrasonic Location Beacon (ULB) transmitters installed in a building. Each ULB transmitter contains both RF transmitter and US (ultrasonic) transmitter. During working, each RF transmitter will emit RF signal if it hears a clean RF channel, and simultaneously the US transmitter will emit ultrasonic signal. A receiver that is carried on an object will firstly receive the RF signal for synchronization with each ULB transmitter, then receive the ultrasonic signal so that it can measure the distance between the transmitter and itself using Time of Arrival (TOA), and then infer its own physical position for navigating when the object receives more than 3 TOA samples. However, there are disadvantages of Cricket system: 1) multiple RF transmission and multiple US transmission are employed in Cricket system so as to make the coordination rather complex and to increase the system cost; 2) since the RF transmitters are randomly selected such that only one ULB transmitter may emit RF and US signal at a time, the RF emission from all ULB transmitters are unordered; and 3) an Object hears only one ultrasonic beacon at a time, and may move between chirps from different beacons. As a result, there is no guaranteed simultaneity in the distance samples, which can lead to inaccurate position estimation.